The ability of enzymes to catalyze biological reactions is fundamental to life. A range of biological applications use enzymes to synthesize various biomolecules in vitro. One particularly useful class of enzymes is the polymerases, which can catalyze the polymerization of biomolecules (e.g., nucleotides or amino acids) into biopolymers (e.g., nucleic acids or peptides). For example, polymerases that can polymerize nucleotides into nucleic acids, particularly in a template-dependent fashion, are useful in recombinant DNA technology and nucleic acid sequencing applications. Many nucleic acid sequencing methods monitor nucleotide incorporations during in vitro template-dependent nucleic acid synthesis catalyzed by a polymerase. Single Molecule Sequencing (SMS) and Paired-End Sequencing (PES) typically include a polymerase for template-dependent nucleic acid synthesis. Polymerases are also useful for the generation of nucleic acid libraries, such as libraries created during emulsion PCR or bridge PCR. Nucleic acid libraries created using such polymerases can be used in a variety of downstream processes, such as genotyping, nucleotide polymorphism (SNP) analysis, copy number variation analysis, epigenetic analysis, gene expression analysis, hybridization arrays, analysis of gene mutations including but not limited to detection, prognosis and/or diagnosis of disease states, detection and analysis of rare or low frequency allele mutations, and nucleic acid sequencing including but not limited to de novo sequencing or targeted resequencing.
When performing polymerase-dependent nucleic acid synthesis or amplification, it can be useful to modify the polymerase (for example via mutation or chemical modification) so as to alter its catalytic properties. In some instances, it can be useful to modify the polymerase to enhance its catalytic properties. Polymerase performance in various biological assays involving nucleic acid synthesis can be limited by the kinetic behavior of the polymerase towards nucleotide substrates. For example, analysis of polymerase activity can be complicated by undesirable behavior such as the tendency of a given polymerase to dissociate from the template; to bind and/or incorporate the incorrect, e.g., non Watson-Crick base-paired, nucleotide; or to release the correct, e.g., Watson-Crick based paired, nucleotide without incorporation. These and other desirable properties can be enhanced via suitable selection, engineering and/or modification of a polymerase of choice. For example, such modification can be performed to favorably alter the polymerase's rate of nucleotide incorporation, affinity of binding to template, processivity or average read length; such alterations can increase the amount of sequence information obtained from a single sequencing reaction. There remains a need in the art for improved polymerase compositions exhibiting altered, e.g., increased processivity, read length (including error-free read length) and/or affinity for DNA template. Such polymerase compositions can be useful in a wide variety of assays involving polymerase-dependent nucleic acid synthesis, including nucleic acid sequencing and production of nucleic acid libraries.